A probe is a device that ideally non-invasively connects a measurement device to a device under test (also referred to as a “DUT”). Known measurement devices, that utilize probes, include: oscilloscopes; spectrum analyzers; network analyzers; logic analyzers; counters; and time interval meters. In such known measurement devices, minimally invasive probes are connected to an indeterminate source of a signal to be tested and a ground of a DUT. To function in a minimally invasive manner, a probe has high impedance relative to the source impedance such that the probe does not add a significant load to the circuit under test. This feature separates probes from standard interconnects, such as used by power supplies and the like.
Active probes are a class of probes that contain active circuits, typically designed to provide impedance buffering near the probe tip to drive a 50 ohm transmission line to the measurement equipment One example of such an active probe is the AGILENT 1158A which is a 4 GHz Active probe for the AGILENT INFINIIUM oscilloscope. Active probes generally comprise three parts: a probe pod with circuits powered by the measuring device; the probe tip for interfacing with the DUT; and a transmission line between the probe tip and pod typically comprising a coax line with one or more single line wires.
A reoccurring problem with probes are dc errors and noise generated by the current flowing through the ground shield of the transmission line used by the probe. Such problems, especially at DC or low frequencies (such as 60 Hz), can be significant. Most probes include instructions for manually checking for these effects by grounding the probe tip and ground tip to the DUT ground to give the user an understanding of the level of these problems.
Various techniques have been used in the past to minimize the ground shield noise. For example, it is known to connect the DUT to an AC supply as close as possible to the measurement equipment. However, this is not always practical and may not limit the current enough for cutting edge measurement equipment. Another solution has been to insert a choke (inductor) into the power line ground of either the DUT or the measurement equipment so as to limit the current flow into the ground. This solution has not proven satisfactory at low frequencies and is not effective at all for DC. One solution that may help for DC is the use of a heavy conductor for the transmission line, unfortunately this solution has not been successful for low frequency signals. One solution that actually works is to float either the DUT or the measurement equipment by cutting the power ground line. Unfortunately, such a solution is quite dangerous.
The present inventors have discovered an apparatus and method for automatically compensating for noise caused by currents flowing in the transmission line ground of a probe.